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Full text of "Text Book Of Mechanical Engineering"

Fairbairn Cranes and Shi/s Davits.

Example 33.—Fig". 415 shews a c Fairbairn' crane. Draw the curve
of bending" moment for all sections, and design a suitable sectioa at
AB, taking/0— 5 tons. (Hons. Mach, Constr. Ex, 1887.)

Bm diagram is given in Fig. 415, using1 centre line of jib as base
line. At each section the

Moment — W x horizontal arm to axis of section.

Section at A B can only be obtained by trial and error, and has thus
been found in Fig. 415.    Checking by approximate method :

Area of two angles, one flange, and )       ,       -
portion of web between angles     j           "'

4 = 384    and total area = 45 sq. ins.

r = 15 x 12 = 180". •
10    10 x 180

•22-+47 = 4-92 tons.

Ships' davits are similarly calculated, but their sections are
like that* of a crane hook, and the same precautions apply.

Strength of Pillars and Struts.—Although these fail
by compression and bending, the action is not so simple. Struts
having a length of ten or twelve times their diameter are reckoned
for direct crushing only, but longer pillars bend before breaking.
Euler* devised a formula to give the greatest load consistent with
stability, that is, beyond which, the bar could not restraighten.

v "

Let Q = —jj—.     Then the stable loads w are given in  the

following table:—

* Pronounced «Oyler/